Good advice above. Try to pick a tree with surrounding cover. It is hard to tell you a specific height but if you're getting busted you must go higher or select trees with better background.

If you are uncomfortable higher then try to time your movements more carefully. A good rule of thumb is that if you can see the deer's eye then it can see you easily. Their peripheral vision is outstanding.

Also, remember that higher stand heights increase the difficulty of double lunging the deer especially at closer ranges. As you go higher, the available target area that will deflate both lungs will present a smaller profile. Double lung = easy recovery. Single lung = long, difficult recovery.